This invention relates to a technology for printing an image on print media using a bi-directional reciprocating movement in a main scanning direction, and more specifically to a technology of bi-directional printing for recording each pixel with a variable-size ink dot.
In recent years color printers that emit colored inks from a print-head are coming into widespread use as computer output devices. Some of these inkjet color printers have the function of xe2x80x98bi-directional printingxe2x80x99, in order to increase the printing speed.
The conventional inkjet printer prints each pixel with two levels, that is, on and off. Multilevel printers have recently been proposed, which prints each pixel with three or more values. The multilevel pixels are formed, for example, by emitting a plurality of ink droplets having an identical color in each one-pixel area.
When bi-directional printing is carried out in the multilevel printer that emits a plurality of ink droplets in each one-pixel area, the hitting positions of ink droplets during the reverse pass are not aligned in the main scanning direction with those during the forward pass. This results in undesirably deteriorating the image quality.
FIG. 31 shows positional deviation of ink droplets in the main scanning direction that occur in bi-directional printing. Each lattice in FIG. 31 represents the boundary of a one-pixel area; one rectangular area defined by the lattice lines corresponds to a one-pixel area. A print head (not shown) moves in the main scanning direction and emits ink droplets to print the respective pixels. In the example of FIG. 31, odd-numbered raster lines L1, L3, and L5 are printed during the forward pass, whereas even-numbered raster lines L2 and L4 are printed during the reverse pass. The amount of ink emitted is regulated for each pixel so that one of three different dots having different sizes can be formed in the one-pixel area. A small dot is formed by emitting a relatively small ink droplet in the one-pixel area, whereas a medium dot is formed by emitting a relatively large ink droplet in the one-pixel area. A large dot is formed by emitting both of the ink droplets for forming a small dot and a medium dot in the one-pixel area. In this way, each pixel can be printed in one of four different tone levels (that is, no dot, small dot, medium dot, and large dot).
As clearly understood from FIG. 31, in the conventional bi-directional printing, the hitting positions of ink droplets during the forward pass of the main scan are different in the main scanning direction from those during the reverse pass. Relatively small ink droplets to form small dots hit on the left half of the one-pixel area in the forward pass, but hit on the right half of the one-pixel area in the reverse pass. Relatively large ink droplets to print medium dots, on the other hand, hit on the right half of the one-pixel area in the forward pass, but hit on the left half of the one-pixel area in the reverse pass. This causes a line, which is expected to extend straight in the sub-scanning direction, to be in zigzag.
As can be understood from the above example, when bi-directional printing is carried out in the conventional inkjet multilevel printer, differences in printing properties between the reverse and forward passes tends to deteriorate the image quality.
The present invention is made to solve the above problem of the prior art, and an object of the present invention is to effectively prevent deterioration of the image quality because of differences in printing properties between the reverse and forward passes in bi-directional printing in an inkjet multilevel printer.
In order to solve at least part of the above problems, the present invention provides a bi-directional printing technique using a printer including a print head having a plurality of nozzles and a plurality of emission driving elements for causing emission of ink droplets respectively from the plurality of nozzles, each nozzle being adaptable to form a selected one of N different dots having different sizes in one pixel area on the print medium, where N is an integer of at least 2. According to the present invention, a shape of the drive signal within each one-pixel period of main scan is modified to have N different waveforms corresponding to N different values of the print signal, the N different values of the print signal representing formation of the N different dots, while changing the N different waveforms of the drive signal between the forward pass and the reverse pass.
The change of the N different waveforms of the drive signal between the forward pass and the reverse pass effectively prevents deterioration of the image quality because the difference in printing properties between the forward pass and the reverse pass. By way of example, this arrangement will align the hitting positions of ink droplets in the main scanning direction in the forward pass and in the reverse pass. This accordingly prevents deterioration of the image quality because of a misalignment of the hitting positions of ink droplets in the main scanning direction.
The drive signal to be supplied to each of the emission driving elements may be generated by: generating an original drive signal having a plurality of pulses within the one-pixel period of main scan, the original drive signal being commonly used for the plurality of emission driving elements; generating N different masking signals corresponding to the N different values of the print signal, in order to selectively mask the plurality of pulses of the original drive signal; and selectively masking the plurality of pulses of the original drive signal with respect to each of the emission driving elements with the masking signals. In this case, waveforms of the N different masking signals corresponding to the N different values of the print signal are changed between the forward pass and the reverse pass. This arrangement will readily modify the waveform of the drive signal in the forward pass and in the reverse pass to have the N different waveforms corresponding to the different values of the print signal.
The waveform of the original drive signal within each one-pixel period of main scan may be changed between the forward pass and the reverse pass. This can modify the waveform of the original drive signal in such a manner as to absorb the difference in printing properties between the forward pass and the reverse pass. selecting one of a plurality of gradient values representing gradients of the waveform of the original drive signal;
The modification of the original drive signal may be attained by: adding the selected gradient value with a fixed period to generate level data representing a level of the original drive signal; carrying out D-A conversion of the level data to generate the original drive signal; and changing the plurality of gradient values between the forward pass and the reverse pass. This arrangement will attain the change of the original drive signal between the forward pass and the reverse pass with a relatively simple structure.
Alternatively, the drive signal waveform may be modified by: generating a plurality of drive signal pulses within each one-pixel period of main scan for emitting the plurality of ink droplets in each one-pixel area on the print medium, while reversing, within each one-pixel period of main scan, supply timing of at least one of the drive signal pulses in the one-pixel period to emit ink droplets, to the emission driving element between the forward pass and the reverse pass. The reversing of the drive signal pulses between the forward and reverse passes will align the hitting positions of ink droplets in the main scanning direction in the forward pass and those in the reverse pass. This effectively prevents deterioration of the image quality because of misalignment of the hitting positions of ink droplets in the main scanning direction.
The drive signal pulses may be generated responsive to a bit-sequence modified signal, which is produced by reversing bit positions in the multi-bit print signal between the forward pass and the reverse pass, thereby producing a bit-sequence modified signal. When the drive signal pulses are reversed between the forward pass and the reverse pass, ink droplets of suitable for recording pixels can be emitted responsive to the bit-sequence modified signal.
The plurality of drive signal pulses may be generated responsive to the bit-sequence modified signal. In this case, the plurality of drive signal pulses are generated as pulses having different waveforms, which are used to emit ink droplets having different amounts of ink, corresponding to the N different values of the print signal. A plurality of tone levels can be expressed in one pixel by emitting or non-emitting a plurality of ink droplets having different amounts of ink. The above arrangement also prevents deterioration of the image quality because misalignment of the hitting positions of ink droplets in the main scanning direction.
Furthermore, a plurality of original drive signal pulses having different waveforms may be generated in each one-pixel period of main scan while reversing generation timings of the plurality of original drive signal pulses within each one-pixel period of main scan between the forward pass and the reverse pass. In this case, the drive signal pulses used for recording each pixel may be generated by masking the plurality of original drive signal pulses with the bit-sequence modified signal.
Alternatively, the drive signal pulses used for recording each pixel may be produced by: generating a plurality of original drive signal pulses having a substantially identical waveform within each one-pixel period of main scan, in order to cause a plurality of ink droplets having a substantially fixed amount of ink to be emitted within each one-pixel period of main scan; and masking the plurality of original drive signal pulses with the bit-sequence modified signal.
The present invention can be embodied in various forms such as a printing method, a printing apparatus, a computer program that has the functions of the method or of the apparatus, a computer readable medium on which is recorded the computer program, and a data signal embodied in a carrier wave comprising the computer program.